1. Field of the Invention
Device for determining a centring error of a round track-shaped information structure in an optical record carrier relative to the axis of rotation of a turntable supporting the record carrier, apparatus provided with such a device, and record carrier intended for use in conjunction with said device.
The invention relates to an optical record carrier adapted for use in scanning apparatus, which includes a device for determining the centering error of a substantially circular information track structure in a surface layer of the substrate of an optical record carrier relative to the axis of rotation of a turntable supporting the record carrier. Such apparatus and device is described in the above U.S. Pat. No. 4,761,776. Such a device device comprises at least one radiation source for projecting a beam of radiation onto the information structure and a radiation-sensitive detection system for converting the resulting radiation thereform into an electric signal which is indicative of the centering error. The apparatus for scanning the record carrier produces an optical radiation beam in order to read and/or record information in the information track structure. This enables the novel record, carrier herein to be put to practical use owing to the centering-errors detection device and associated positioning means for positioning the centre of the track structure relative to the axis of rotation of the turntable.
The circular information track structure comprises a succession of adjacent concentric tracks, or a succession of quasi-concentric continuous tracks which together constitute a spiral track. These tracks may be inscribed partly or wholly with information, but alternatively they may be largely blank tracks in which a user can record the desired information. During information recording the blank tracks are also used as servo tracks in order to ensure that a write radiation spot accurately follows a specific path. A centring error is to be understood to mean the deviation, in two directions, between the centre of the concentric tracks or the spiral track and the axis of rotation of the turntable supporting the record carrier.
2. Description of the Related Art.
Optical record carriers in the form of disc-shaped records containing, for example, a video program or an audio program, are known as "Laservision Disc" and "Compact Disc" ("CD") respectively apparatus for reading these record carriers has been described frequently and comprehensively. By way of example reference is made to "Philips Technical Review", Vol. 33, No. 7, pages 178-193 and Vol. 40, No. 6, pages 149-155. An optical record carrier in which a user can record the desired information is described in: "IEEE Spectrum", August 1979, pages 26-33.
All the known ready-for-use record carriers have a centre hole by which they are slid onto a spindle when the record carrier is loaded into a write or read apparatus, so that the record carrier is fixed on a spindle and can be rotated by means of the spindle. The centre of the hole should very accurately coincide with the centre of the circular tracks or the spiral track and the diameter of this hole should be equal to the diameter of the spindle with a very high accuracy in order to ensure that during rotation of the record carriers the tracks practically do not move eccentrically or, in other words, practically do not wobble relative to the scanning unit. The read and write apparatus comprises a tracking system by means of which a deviation between the centre of the scanning spot and the centre line of a track being scanned can be detected and eliminated. This tracking system is intended for eliminating comparatively small tracking errors which arise as a result of vibrations in the read or write apparatus, or as a result of small deviations in the track spacings, etc. The tracking system can also eliminate tracking errors caused by a centering error but this is possible only to a limited extent, for example up to centering errors of the order of 100 .mu.m. However, in practice the known record carriers may exhibit larger centering errors as a result of manufacturing tolerances. For example, the position of the centre of the hole relative to the centre of the track structure is subject to specific tolerances and the hole may exhibit out-of-roundness. Moreover, in the case of frequent use of the record carrier the hole therein may be subjected to wear, so that the record carrier is not tightly mounted on the spindle. For these reasons it is required to have a device by means of which such errors can be detected and corrected during reading and/or recording.
This need becomes even greater for novel uses of the optical track-shaped information structure, for example as a storage means in a disc or record which in addition to the optical storage means is provided with electronic or magnetic storage means, which disc or record may not be formed with a hole.
In the case of the known record carriers such as the "Compact Disc" the "Laservision Disc" and the digital optical disc of the direct-recording type referred to as "D.O.R." disc it may be advantageous if the record carrier need not be formed with a hole. This is not only because it is difficult to form this hole exactly at the desired location but also because stresses may be produced in the record-carrier material when this hole is formed, which stresses may give rise to changes in the optical properties of this material, which may impair a correct read-out or recording of the information structure.
In a customary record carrier of the direct-recording type the information layer is enclosed in an airtight manner between the substrate in which the information structure is formed and a disc which is spaced from the back of the substrate by an interposed spacer. If this record carrier is formed with a hole this hole should also be sealed, which is a difficult task.
If the known audio or video record carriers are to be constructed as double record carriers, i.e. record carriers having two reflecting information layers arranged back-to-back, forming a centre hole becomes even more difficult because this hole must be centred relative to the two track structures. In particular for a record carrier having two information layers and for the above-mentioned direct-recording type record carrier it is very advantageous if no hole is to be formed in the record carrier.
When the known audio or video record carriers with centre holes are manufactured these holes are generally formed after the track structure has been formed. It is then necessary to ascertain where the centre of the track structure is located, so that the tool by means of which the hole is formed can be centred relative to this centre. U.S. Pat. No. 3,915,576 describes an apparatus for this purpose. In this apparatus the entire record carrier is illuminated with a broad beam and the radiation originating from this record carrier is received by a single detector arranged behind a diaphragm having a small aperture. This detector receives only radiation originating from the track structure. The radiation originating from the trackless central portion of the information layer is stopped by an absorbing plate. The record carrier is positioned in such a way that a maximum amount of radiation is incident on the detector. The centre of the track structure is then situated on the line connecting the radiation source to the detector. In the apparatus in accordance with U.S. Pat. No. 3,915,576 the detector signal must be compared with a reference value. Moreover, it is possible only to ascertain whether the centre of the track structure is situated in line with the radiation source and the detector but it is not possible to determine the sign of a centering error. In order to determine this sign additional steps are necessary, which are not described in said Patent Specification.